how will we express the result, will we be asked to write the solution on the base grid? Will we have to complete the yarns as well? Will we be allowed to make markings, candidates etc on the base grid?

Solution on base grid is all that is required for credit for the puzzle. However if that is incorrect or incomplete, partial points will be awarded as specified in the IB for any yarn correctly marked with the row or column it goes into for the base grid.
Any markings and candidates and so on are allowed as in any normal round.